Rear End - Differential

I recently had the rear end replaced due to a bad diff (as per mechanic). There was a constant noise coming from the rear when I accelerated. Now I'm getting a thumping sound from the rear when I release the brake after stopping. I've been back and forth w/ the mechanic as he does not seem to know what the problem is. First he thought and still thinks its the brakes not releasing properly. I think it may be a bad rear end from the salvage.

Can anyone provide guidance as it's getting riduculous going back and forth to the mechanic

I believe I've got the same problem on my Safari 2002 AWD. One mechanic said that he thinks it has to do with a clutch-type plate in the rear differential which is slipping, creating a kind of thumping under acceleration and even deceleration. His solution is to open up the differential, replace the fluid and add a prescribed GMC additive that will reduce the problem although not completely solve it, at a cost of approximately $125. Apparently he had to do the same job on a RendezVous.If there's another more accurate analysis of to this problem, I'd like to hear it.By the way, how can I access the discussion #1636 mentioned in the # 3 box above?

i have an 01 astro i just bought it dose the same thing i had an 05 tahoe that did that and i took it in and they said it was the drive shaft that it needed to be repacked with grease so they did it and it stopped for a few months and stated again